System and method for wide band Raman amplification

ABSTRACT

A multi-stage Raman amplifier includes a first Raman amplifier stage having a first sloped gain profile operable to amplify a plurality of signal wavelengths, and a second Raman amplifier stage having a second sloped gain profile operable to amplify at least most of the plurality of signal wavelengths after those wavelengths have been amplified by the first stage. The second sloped gain profile is approximately complementary slope to the slope of the first sloped gain profile. The combined effect of the first and second Raman stages contributes to an approximately flat overall gain profile over the plurality of signal wavelengths.

STATEMENT OF OTHER APPLICATIONS

This application discloses subject matter that is in some respectssimilar to that disclosed in copending application Ser. No. 09/817,312,entitled Method and System for Reducing Degradation of Optical Signal toNoise Ratio, filed Mar. 16, 2001.

This application also discloses subject matter that is in some respectssimilar to that disclosed in copending application Ser. No. 09/768,367,entitled All Band Amplifier, filed Jan. 22, 2001. Application Ser. No.09/768,367 is a continuation-in-part of U.S. application Ser. No.09/719,591, filed Dec. 12, 2000, which claims the benefit of copendingapplication Ser. No. PCT/US99/13551, entitled Dispersion Compensatingand Amplifying Optical Element, Method for Minimizing Gain Tilt andApparatus for Minimizing Non-Linear Interaction Between Band Pumps,filed on Jun. 16, 1999, and published on Dec. 23, 1999 as WO 99/66607,which in turn claims the benefit of U.S. application Ser. No.60/089,426.

This U.S. application Ser. No. 09/811,103 and U.S. application Ser. No.09/768,367 are currently assigned to Xtera Communications, Inc.

TECHNICAL FIELD OF THE INVENTION

The present invention relates to the field of communication systems, andmore particularly to a system and method operable to facilitate wideband optical amplification while maintaining acceptable noise figures.

BACKGROUND OF THE INVENTION

Because of the increase in data intensive applications, the demand forbandwidth in communications has been growing tremendously. In response,the installed capacity of telecommunication systems has been increasingby an order of magnitude every three to four years since the mid 1970s.Much of this capacity increase has been supplied by optical fibers thatprovide a four-order-of-magnitude bandwidth enhancement overtwisted-pair copper wires.

To exploit the bandwidth of optical fibers, two key technologies havebeen developed and used in the telecommunication industry: opticalamplifiers and wavelength-division multiplexing (WDM). Opticalamplifiers boost the signal strength and compensate for inherent fiberloss and other splitting and insertion losses. WDM enables differentwavelengths of light to carry different signals in parallel over thesame optical fiber. Although WDM is critical in that it allowsutilization of a major fraction of the fiber bandwidth, it would not becost-effective without optical amplifiers. In particular, broadbandoptical amplifier systems that permit simultaneous amplification of manyWDM channels are a key enabler for utilizing the full fiber bandwidth.

Traditionally, amplification of signals having a broad range ofwavelengths has required separating the signals into subsets ofwavelengths, and amplifying each subset with a separate amplifier. Thisapproach can be complex and expensive. Using separate amplifiers foreach subset requires additional hardware, additional laser pumps foreach amplifier, and additional power to launch the additional pumps.

Although a more efficient approach would be to amplify the entire signalusing a single amplifier for at least some amplifiers in the system,unfortunately, no acceptable single amplifier approach has beendeveloped. For example, erbium doped-amplifiers are an inherently badchoice for wide band amplification if the ultimate goal is to provide anamplifier that can operate over the entire telecommunications spectrum.For example, for wavelengths shorter than about 1525 nanometers,erbium-atoms in typical glasses will absorb more than they amplify. Evenwith use of various dopings, such as, aluminum or phosphorus, theabsorption peak for the various glasses is still around 1530 nanometers.This leaves a large gap in the short communications band (S-Band)unreachable by erbium doped fiber amplifiers.

Raman amplifiers provide a better solution in terms of broadbandamplification potential, but conventional Raman amplifiers have sufferedfrom other shortcomings. For example, Raman amplifiers havetraditionally suffered from high noise figures when used in wide bandapplications. In addition, Raman amplifiers suffer from gain tiltintroduced when longer wavelength signals rob energy from shorterwavelength signals. This effect becomes increasingly pronounced asamplifier launch power and system bandwidth increases. Wide band Ramanamplifiers operating at high launch powers on a wide range ofwavelengths can be particularly vulnerable to this effect.

Masuda, et al. (see e.g., U.S. Pat. No. 6,172,803 B1 and relatedresearch papers) have attempted to improve the bandwidth of erbium dopedamplifiers by cascading with the erbium doped amplifier a Ramanamplifier with an approximately complementary gain profile. Masuda, etal, however, consistently require the presence of an erbium dopedamplifier (which relies on different physics for amplification and doesnot suffer from the same noise problems as Raman amplifiers do) toprovide virtually all amplification to signal wavelengths close inspectrum to the pump wavelengths. Indeed, Masuda, et al. concede thatthe noise figures they report ignore the effect of the Raman portion oftheir amplifier.

SUMMARY OF THE INVENTION

The present invention recognizes a need for a method and apparatusoperable to facilitate wide band Raman amplification while maintainingan approximately flat gain profile and an acceptable noise figure.

In accordance with the present invention, a system and method forproviding wide band Raman amplification are provided that substantiallyreduce or eliminate at least some of the shortcomings associated withprior approaches. In one aspect of the invention, a multi-stage Ramanamplifier comprises a first Raman amplifier stage having a first slopedgain profile operable to amplify a plurality of signal wavelengths, anda second Raman amplifier stage having a second sloped gain profileoperable to amplify at least most of the plurality of signal wavelengthsafter those wavelengths have been amplified by the first stage. Thesecond sloped gain profile has an approximately complementary slope tothe slope of the first sloped gain profile. The combined effect of thefirst and second Raman stages contributes to an approximately flatoverall gain profile over the plurality of signal wavelengths.

In another aspect of the invention, a method of amplifying an opticalsignal having multiple wavelengths comprises amplifying a plurality ofsignal wavelengths at a first Raman amplifier stage having a firstsloped gain profile, and amplifying at least most of the plurality ofsignal wavelengths at a second Raman amplifier stage after those signalwavelengths have been amplified by the first stage. The second stage hasa second sloped gain profile comprising an approximately complimentarygain profile to the first gain profile. The combined effect of the firstand second Raman stages contributes to an approximately flat overallgain profile over the plurality of signal wavelengths.

In still another aspect of the invention, a multi-stage Raman amplifiercomprises a plurality of cascaded Raman amplifier stages each having again profile, wherein the gain profile of at least some of the Ramanstages is sloped. At least two of the sloped gain profiles compriseapproximately complimentary gain profiles, wherein the combined effectof the gain profiles of the amplification stages results in anapproximately flat overall gain profile over a plurality of signalwavelengths amplified by the amplifier.

In yet another aspect of the invention, a method of amplifyingmultiple-wavelength optical signals comprises applying a first slopedgain profile to a plurality of signal wavelengths at a first stage of aRaman amplifier, and applying a second sloped gain profile to at leastmost of the plurality of signal wavelengths at a second stage of theRaman amplifier. The second gain profile comprises an approximatelycomplementary gain profile of the first sloped gain profile. Thecombined effect of the first and second sloped gain profiles contributesto an approximately flat overall gain profile over the plurality ofsignal wavelengths.

In another aspect of the invention, a multi-stage Raman amplifiercomprises a plurality of cascaded Raman amplifier stages each operableto amplify a plurality of signal wavelengths and each having a gainprofile determined at least in part by one or more pump wavelengthsapplied to the amplifier stage. The plurality of amplifier stagescomprise a first Raman stage operable to apply a higher gain level to asignal wavelength closest to a longest pump wavelength than a gainapplied to a signal wavelength furthest from the longest pumpwavelength.

In still another aspect of the invention, a method of amplifying anoptical signal having multiple wavelengths comprises receiving aplurality of signal wavelengths at a plurality of cascaded Ramanamplifier stages having at least a first stage and a last stage, whereeach stage is operable to amplify a plurality of signal wavelengths andeach stage has a gain profile determined at least in part by one or morepump wavelengths applied to the amplifier stage. The method furtherincludes applying a highest level of gain supplied by the longest pumpwavelength in the last Raman stage of the amplifier.

In yet another aspect of the invention, a multi-stage Raman amplifiercomprises a plurality of cascaded Raman amplifier stages, at least someof the Raman stages having sloped gain profiles operable to contributeto a combined gain profile of the amplifier. The combined gain profileof the amplifier is approximately flat across a bandwidth of at leasteighty nanometers and comprises a small signal noise figure no greaterthan eight decibels.

In another aspect of the invention, a method of amplifying an opticalsignal having multiple wavelengths comprises amplifying a plurality ofsignal wavelengths at a first Raman amplifier stage having a firstsloped gain profile, and amplifying at least most of the plurality ofsignal wavelengths at a second Raman amplifier stage having a secondsloped gain profile that is different than the first sloped gainprofile. The combined gain profile of the amplifier is approximatelyflat across a bandwidth of at least eighty nanometers and comprises asmall signal noise figure no greater than eight decibels.

In another aspect of the invention, an optical pre-amplifier operable tobe coupled to an optical communication link carrying optical signalshaving a plurality of wavelengths comprises a first Raman stage having again profile where a majority of shorter signal wavelengths areamplified more than a majority of longer signal wavelengths. Thepreamplifier further comprises a second Raman stage operable to receiveat least most of the signal wavelengths after they have been amplifiedby the first stage, the second stage having a gain profile where amajority of longer signal wavelengths are amplified more than a majorityof shorter signal wavelengths. In this embodiment, the gain profiles ofthe first and second Raman stages are operable to combine to contributeto an approximately flat combined gain profile over the plurality ofsignal wavelengths.

In still another aspect of the invention, an optical booster amplifieroperable to be coupled to an optical communication link carrying opticalsignals having a plurality of wavelengths comprises a first Raman stagehaving a gain profile where a majority of longer signal wavelengths areamplified more than a majority of shorter signal wavelengths. Thebooster amplifier also comprises a second Raman stage operable toreceive at least most of the signal wavelengths after they have beenamplified by the first stage, the second stage having a gain profilewhere a majority of shorter signal wavelengths are amplified more than amajority of longer signal wavelengths. The gain profiles of the firstand second Raman stages are operable to combine to contribute to anapproximately flat combined gain profile over the plurality ofwavelengths.

In yet another aspect of the invention, a Raman amplifier assemblycomprises a preamplifier coupled to an optical communication link. Thepreamplifier includes a first Raman stage having a gain profile whereina majority of shorter wavelengths are amplified more than a majority oflonger wavelengths, and a second Raman stage having a gain profileapproximately complementary to the first gain stage. The amplifierassembly also includes a booster amplifier coupled to the opticalcommunication link. The booster amplifier comprises a first Raman stagehaving a gain profile wherein a majority of longer wavelengths areamplified more than a majority of shorter wavelengths, and a secondRaman stage having a gain profile approximately complementary to thefirst gain stage.

In another aspect of the invention, an optical communication systemoperable to facilitate communication of multiple signal wavelengthscomprises a transmitter bank operable to generate a plurality of signalwavelengths, and a multiplexer operable to combine the plurality ofsignal wavelengths into a single multiple wavelength signal fortransmission over a transmission medium. The system further comprises anamplifier coupled to the transmission medium and operable to amplify themultiple wavelength signal prior to, during, or after the multiplewavelength signal's transmission over the transmission medium, theamplifier comprising a multi-stage Raman amplifier. The amplifierincludes a first Raman amplifier stage having a first sloped gainprofile operable to amplify a plurality of signal wavelengths and asecond Raman amplifier stage having a second sloped gain profileoperable to amplify at least most of the plurality of signal wavelengthsafter those wavelengths have been amplified by the first stage. Thesecond sloped gain profile has an approximately complementary slope tothe slope of the first sloped gain profile, and the combined effect ofthe first and second Raman stages contributes to an approximately flatoverall gain profile over the plurality of signal wavelengths. In oneembodiment, the system further includes a demultiplexer operable toreceive the multiple wavelength signal and to separate the signalwavelengths from the multiple wavelength signal, and a receiver bankoperable to receive the plurality of signal wavelengths.

Depending on the specific features implemented, particular embodimentsof the present invention may exhibit some, none, or all of the followingtechnical advantages. For example, one aspect of the inventionfacilitates optical amplification of a wide bandwidth of wavelengthswhile maintaining an approximately flat gain profile and an acceptablenoise figure.

In a particular embodiment, one aspect of the invention reduces thenoise figure associated with the amplifier by amplifying in a firstRaman stage a majority of shorter wavelengths more than a majority oflonger wavelengths. In this way, shorter wavelengths (which are oftenclosest to the pump wavelength) are amplified to overcome any effectsthat might be caused by phonon-stimulated noise. As a furtherenhancement, the amplifier could be designed so that the longest pumpwavelength is at least ten nanometers below the shortest signal beingamplified.

In addition to yielding an acceptable noise figure, this approach canproduce an approximately flat gain tilt, for example, by cascading asecond Raman amplifier stage having a gain profile that amplifies amajority of longer wavelengths more than a majority of shorterwavelengths. In a particular embodiment, the second gain profile can beapproximately complementary to the first gain profile. In someapplications, the second gain profile can have an approximately equal(although opposite) slope from the first gain profile.

Another aspect of the invention results in increased efficiency in amulti-stage Raman amplifier. This aspect of the invention involvesapplying, in at least one Raman stage, a first gain profile thatamplifies a majority of longer wavelengths more than a majority ofshorter wavelengths; and applying, in a later cascaded Raman stage, asecond gain profile that amplifies a majority of shorter wavelengthsmore than a majority of longer wavelengths. This embodiment facilitatesallowing longer pump wavelengths in the first stage to accept energyfrom shorter pump wavelengths in the later Raman stage. This effect, inturn, facilitates using smaller pump wavelengths and/or fewer pumpwavelengths in the first stage than would otherwise be required, therebyincreasing the efficiency of the device. In a particular embodiment, thegain profiles of the first and later Raman stages can be approximatelycomplimentary, contributing to an approximately flat overall gainprofile for the amplifier. The noise figure can be reduced, for example,by performing a majority of the amplification of wavelengths closest tothe pump wavelengths in one of the final amplifier stages, or in thelast amplifier stage.

Other aspects of the invention facilitate cascading multiple amplifierstages to realize advantages of low noise and high efficiency in amultiple stage Raman amplifier. Moreover, cascaded stages can providemid-stage access to the amplifier to facilitate, for example, opticaladd/drop multiplexing of WDM signals while maintaining an acceptablenoise figure and an approximately flat gain profile, both at themid-stage access point and across the entire amplifier.

Other technical advantages are readily apparent to one of skill in theart from the attached figures, description, and claims.

BRIEF DESCRIPTION OF THE DRAWINGS

For a more complete understanding of the present invention, and forfurther features and advantages thereof, reference is now made to thefollowing description taken in conjunction with the accompanyingdrawings, in which:

FIG. 1 is a block diagram showing an exemplary optical communicationsystem operable to facilitate communication of wide band optical signalsconstructed according to the teachings of the present invention;

FIG. 2 is a graphical illustration of the phonon-stimulated opticalnoise figure;

FIG. 3a is a block diagram of an exemplary embodiment of a multiplestage Raman amplifier constructed according to the teachings of thepresent invention;

FIGS. 3b-3 c show gain profiles associated with various amplificationstages and an overall gain profile for the amplifier shown in FIG. 3a,respectively, constructed according to the teachings of the presentinvention;

FIG. 4a is a block diagram of an exemplary embodiment of a multiplestage Raman amplifier constructed according to the teachings of thepresent invention;

FIGS. 4b-4 c show gain profiles associated with various amplificationstages and an overall gain profile for the amplifier shown in FIG. 4a,respectively, constructed according to the teachings of the presentinvention;

FIG. 5a is a block diagram of an exemplary embodiment of a three stageRaman amplifier constructed according to the teachings of the presentinvention;

FIGS. 5b-5 c show gain profiles associated with various amplificationstages and an overall gain profile for the amplifier shown in FIG. 5a,respectively, constructed according to the teachings of the presentinvention;

FIG. 6a is a block diagram of an exemplary embodiment of a four stageRaman amplifier constructed according to the teachings of the presentinvention;

FIGS. 6b-6 c show gain profiles associated with various amplificationstages and an overall gain profile for the amplifier of FIG. 6a,respectively, constructed according to the teachings of the presentinvention;

FIG. 7 is a flow chart illustrating one example of a method ofamplifying a plurality of wavelengths using a multi-stage Ramanamplifier according to the teachings of the present invention;

FIGS. 8a-8 b show simulated gain and noise profiles for one embodimentof a multi-stage hybrid Raman amplifier constructed according to theteachings of the present invention; and

FIGS. 9a-9 b show simulated gain and noise profiles for one embodimentof a multi-stage discrete Raman amplifier constructed according to theteachings of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 is a block diagram showing an exemplary optical communicationsystem 10 operable to facilitate communication of wide band opticalsignals. System 10 includes a transmitter bank 12 operable to generate aplurality of wavelength signals 16 a-16 n. Transmitter bank 12 mayinclude, for example, a plurality of laser diodes or semiconductorlasers. Each wavelength signal 16 a-16 n comprises at least onewavelength of light unique from wavelengths carried by other signals 16.

System 10 also includes a combiner 14 operable to receive multiplesignal wavelengths 16 a-6 n and to combine those signal wavelengths intoa single multiple wavelength signal 16. As one particular example,combiner 14 could comprise a wavelength division multiplexer (WDM). Theterm wavelength division multiplexer as used herein may includeconventional wavelength division multiplexers or dense wavelengthdivision multiplexers.

In one particular embodiment, system 10 may include a booster amplifier18 operable to receive and amplify wavelengths of signal 16 a prior tocommunication over a transmission medium 20. Transmission medium 20 cancomprise multiple spans 20 a-20 n of fiber. As particular examples,fiber spans 20 could comprise standard single mode fiber (SMF),dispersion-shifted fiber (DSF), non-zero dispersion-shifted fiber(NZDSF), or other fiber type or combinations of fiber types.

Where communication system 10 includes a plurality of fiber spans 20a-20 n, system 10 can include one or more inline amplifiers 22 a-22 m.Inline amplifiers 22 reside between fiber spans 20 and operate toamplify signal 16 as it traverses fiber 20.

Optical communication system 10 can also include a preamplifier 24operable to receive signal 16 from a final fiber span 20 n and toamplify signal 16 prior to passing that signal to a separator 26.Separator 26 may comprise, for example, a wavelength divisiondemultiplexer (WDM), which can operate on wavelength divisionmultiplexed signals or dense wavelength division multiplexed signals.Separator 26 operates to separate individual wavelength signals 16 a-6 nfrom multiple wavelength signal 16. Separator 26 communicates individualsignal wavelength 16 a-6 n to a bank of receivers 28.

At least one amplifier in system 10 comprises a wide band multi-stageRaman amplifier operable to receive a wide bandwidth of wavelengthsignal 16. In a particular embodiment, the amplifier can process over 80nanometers of bandwidth, and in some cases over 100 nanometers ofbandwidth while maintaining an approximately flat gain profile over thebandwidth of amplified signal wavelengths 16.

Throughout this document, the term “approximately flat” describes acondition where the maximum signal gain differs from the minimum signalgain by an no more than amount suitable for use in telecommunicationsystems. The deviation between minimum and maximum signal gains maycomprise, for example five decibels prior to application of any gainflattening filters. Particular embodiments of the invention may achievegain flatness of approximately three decibels prior to application ofany gain flattening filters.

Some amplifiers in system 10 could comprise a plurality of individualamplifiers working in conjunction, each amplifying a subset of thebandwidth processed by the single wide band amplifier. Alternatively,all amplifiers in system 10 could comprises wide bandwidth amplifiers.Depending on the overall bandwidth communicated by system 10, one ormore amplifier locations in system 10 could comprise a plurality of wideband amplifiers operating in conjunction to handle a total bandwidthsignificantly in excess of 100 nanometers. In other cases, a single wideband amplifier could process all traffic at a given location in system10.

Wide band amplifiers within system 10 comprise multi-stage Ramanamplifiers having at least two stages with approximately complimentarygain profiles. A combination of the complimentary gain profiles, incooperation with any other gain stages in the wide band amplifier,results in approximately flat gain profile for the amplifier.

Throughout this description, the phrase “approximately complementary”refers to a situation where, at least in general, signal wavelengths 116that are highly amplified in the first stage are less amplified in thesecond stage, and signal wavelengths 116 that are highly amplified inthe second stage are less amplified in the first stage. Two gainprofiles said to be “approximately complementary” need not have equaland opposite slopes. Moreover, equal amplification of any particularwavelengths in both gain profiles does not preclude those gain profilesfrom being “approximately complementary.”

Conventional designs of multi-stage Raman amplifiers have been unable toprocess bandwidths in excess of 80 nanometers while maintainingapproximately flat gain profiles and acceptable noise figures. Oneaspect of this invention recognizes that a major culprit in noisefigures associated with conventional multi-stage Raman amplifiers is thephonon-stimulated optical noise created when wavelength signals beingamplified reside spectrally close to pump wavelengths used foramplification. One aspect of the invention reduces adverse effect ofthis noise by enhancing the Raman amplification of signal wavelengthsnear the pump wavelengths to overcome the effects of the noise, andapplying an approximately complementary Raman gain profile in anotherstage to result in an approximately flat overall gain profile.

FIG. 2 graphically illustrates the phonon-stimulated optical noisefigure increase as the spectral spacing between signal wavelengths andpump wavelengths decreases. As shown in FIG. 2, phonon-stimulated noiseincreases dramatically as signal wavelength get close to the pumpwavelengths.

One aspect of the invention significantly reduces adverse effectsassociated with phonon-stimulated noise by providing multiple stages ofRaman gain having approximately complimentary gain profiles acting onsubstantially the same bandwidth of signals. While best results areobtained by applying approximately complimentary gain profiles to all ornearly all of the same signal wavelengths, some portion of wavelengthscan be omitted from one gain profile and included in the other gainprofile without departing from the scope of this invention.

FIG. 3a is a block diagram of an exemplary embodiment of a multiplestage Raman amplifier 110 including gain profiles 30 and 40 associatedwith various amplification stages and an overall gain profile 50 for theamplifier. In this example, amplifier 100 comprises a two-stageamplifier having a first stage 112 and a second stage 114 cascaded withfirst stage 112. As will be further discussed below, the invention isnot limited to a particular number of amplifier stages. For example,additional amplification stages could be cascaded onto second stage 114.Moreover, although the illustrated embodiment shows second stage 114cascaded directly to first stage 112, additional amplification stagescould reside between first stage 112 and second stage 114 withoutdeparting from the scope of the invention.

Amplifier 100 could comprise a distributed Raman amplifier, a discreteRaman amplifier, or a hybrid Raman amplifier which comprises bothdiscrete and distributed stages. Each stage 112, 114 of amplifier 100includes an input operable to receive a multiple wavelength opticalinput signal 116. As a particular example, optical input signal 116could include wavelengths ranging over one hundred nanometers.

Each stage 112, 114 also includes distributed gain media 120, 121.Depending on the type of amplifier being implemented, media 120, 121 maycomprise, for example a transmission fiber, or a gain fiber such as aspooled gain fiber. In a particular embodiment, media 120, 121 maycomprise a dispersion compensating fiber.

Each stage 112, 114 further includes one or more wavelength pumps 122.Pumps 122 generate pump light 124 at specified wavelengths, which arepumped into distributed gain media 120, 121. Raman gain results from theinteraction of intense light from the pumps with optical phonons insilica fibers. The Raman effect leads to a transfer of energy from oneoptical beam (the pump) to another optical beam (the signal). Pumps 122may comprise, for example, one or more laser diodes. Although theillustrated embodiment shows the use of counter propagating pumps, undersome circumstances using a relatively quiet pump, co-propagating pumpscould also be used without departing from the scope of the invention.

In one particular embodiment, pump wavelengths 124 can be selected sothat the longest wavelength pump signal 124 has a wavelength that isshorter than the shortest wavelength of signal 116. As one specificexample, the longest wavelength of pump light 124 could be selected tobe, for example, at least ten nanometers shorter than the shortestwavelength of signal 116. In this manner, amplifier 100 can help toavoid phonon stimulated noise that otherwise occurs when pumpwavelengths interact with wavelengths of the amplified signal.

Couplers 118 b and 118 c couple pump wavelengths 124 a and 124 b to gaindistributed media 120 and 121, respectively. Couplers 118 couldcomprise, for example, wave division multiplexers (WDM) or opticalcouplers. A lossy element 126 can optionally reside between amplifierstages 112 and 114. Lossy element 126 could comprise, for example, anisolator, an optical add/drop multiplexer, or a gain equalizer.

The number of pump wavelengths 124, their launch powers, their spectraland spatial positions with respect to other pump wavelengths and otherwavelength signals, and the bandwidth and power level of the signalbeing amplified can all contribute to the shape of the gain profile forthe respective amplifier stage. FIG. 3b shows exemplary gain profilesfor first stage 112 and second stage 114. Gain profile 30 shows theoverall gain of first stage 112 of amplifier 100 for a bandwidth rangingfrom the shortest wavelength of signal 116 (λ_(sh)) to the longestwavelength of signal 116 (λ_(lg)). Gain profile 40 shows the overallgain of second stage 112 of amplifier 100 for a bandwidth ranging fromthe shortest wavelength of signal 116 (λ_(sh)) to the longest wavelengthof signal 116 (λ_(lg)). Each of gain profiles 30 and 40 reflects theeffects of the other gain profile acting upon it.

In this example, gain profile 30 of first stage 112 has a downwardslope, where a majority of the shorter signal wavelengths 116 areamplified more than a majority of the longer signal wavelengths 116.Conversely, gain profile 40 of second stage 114 is approximatelycomplimentary to gain profile 30 of first stage 112. Gain profile 40exhibits an upward slope where a majority of the longer signalwavelengths 116 are amplified more than a majority of the shorter signalwavelengths 116.

Gain profile 50 (shown in dotted lines in FIG. 3c) represents anexemplary composite gain profile of amplifier 100 resulting from theapplication of gain profiles 30 and 40 to optical signal 116. Gainprofile 50 is approximately flat over at least substantially all of thebandwidth of wavelengths within signal 116.

In operation, amplifier 100 receives optical input signal 116 atdistributed gain medium 120 of first stage 112. Distributed gain medium120 could comprise, for example, a dispersion compensating Raman gainfiber, a transmission fiber, a high non-linearly fiber, a segment oftransmission fiber, or combination thereof. Pumps 122(a) generate pumpwavelengths 124(a) and apply them to distributed gain medium 120 throughcoupler 118(b). Pump wavelengths 124 interact with signal wavelengths116, transferring energy from the pump wavelengths 124 to the signalwavelengths 116. In this example, shorter signal wavelengths 116 areamplified more than longer signal wavelengths 116 in first stage 112.

Amplified wavelengths of signal 116 are communicated to distributed gainmedium 121 of second stage 114. Wavelengths of signal 116 are amplifiedin second stage 114 by interacting with pump wavelengths 124 b generatedat pumps 122 b. In this example, pump wavelengths 124 b operate toresult in gain profile 40 where longer wavelengths of signal 116 areamplified more than shorter wavelengths of signal 116.

The combined effect of amplification in first stage 112 and second stage114 of amplifier 100 results in approximately flat gain profile 50across wavelengths of optical signal 116. This particular exampleprovides a significant advantage in reducing the noise figure associatedwith the amplifier. Using this configuration, the small signal noisefigure of amplifier 100 can be reduced to less than eight decibels, insome cases 7 decibels, even where the bandwidth of signal 16 exceeds 100nanometers.

FIG. 4a is a block diagram of another embodiment of a multiple stageRaman amplifier 110 including exemplary gain profiles 130 and 140associated with various amplification stages and an overall gain profile150 for the amplifier. Amplifier 110 shown in FIG. 4 is similar instructure and function to amplifier 100 shown in FIG. 1. Like amplifier100 shown in FIG. 1, amplifier 110 of FIG. 4 includes a first Ramanamplification stage 112 and a second Raman amplification stage 114. Eachof stages 112 and 114 includes a distributed gain medium 120, 121,respectively, which is operable to receive multiple wavelength inputsignal 116 and pump wavelengths 124 a and 124 b, respectively. Eachamplifier stage 112 and 114 operates to amplify wavelengths of signal116 according to gain profiles 130 and 140 as shown.

The example shown in FIG. 4 differs from the example shown in FIG. 3 inthat gain profile 130 (shown in FIG. 4b) of first stage 112 exhibits anupward slope where a majority of longer wavelengths of signal 116 areamplified more than the majority of shorter wavelengths of signal 116.Conversely, gain profile 140 of second stage 114 comprises anapproximately complementary gain profile to first gain profile 130 offirst stage 112. In profile 140 applies a higher gain to a majority ofshorter wavelengths than the gain applied to the majority of longersignal wavelengths 116. In addition, in this embodiment, the launchpower of pumps 122 a driving first gain profile 130 can be reduced.

This aspect of the invention recognizes that due to the Raman scatteringeffect, longer wavelength signals tend to rob energy from shorterwavelength signals. This aspect of the invention leverages that fact toallow the longer pump wavelengths of wavelengths 124 a to rob energyfrom the shorter pump wavelengths of wavelengths 124 b. In a particularembodiment, amplifier 110 may include a shunt 160 between seconddistributed gain medium 121 and first distributed gain medium 120 tofacilitate the longer pump wavelengths of wavelengths 124 a acceptingpower from the shorter pump wavelengths of wavelengths 124 b. Theeffects result in an overall gain profile 130 for first stage 112 thatremains approximately complimentary to the gain profile of second stage140. As a result, the composite gain profile 150 (FIG. 4c) of theamplifier remains approximately flat.

This embodiment provides significant advantages in terms of efficiencyby allowing the use of fewer wavelength pumps 122 a in the first stage112, and/or also by allowing each pump 122 a to operate at a lowerlaunch power.

The embodiment shown in FIG. 4a can also provide improvements for thenoise figure of the amplifier. For example, phonon stimulated noise iscreated in Raman amplifiers where wavelengths being amplified spectrallyreside close to a wavelength of pump signals 124. One aspect of thisinvention recognizes that by spectrally separating pump wavelengths 124from signal wavelengths 116, phonon stimulated noise can be reduced.

In a particular embodiment, pump wavelengths 124 are selected to havewavelengths at least 10 nanometers shorter than the shortest wavelengthin optical signal 116 being amplified. Moreover, in a particularembodiment, second stage 114 where a majority of the gain to shortwavelength of signal 116 is applied comprises the last stage ofamplifier 110.

FIG. 5a is a block diagram of a three stage Raman amplifier 200including gain profiles 230, 240, and 245 associated with variousamplification stages, and an overall gain profile 250 for the amplifier.Amplifier 200 is similar in structure and function to amplifier 100 ofFIG. 3 but includes three cascaded amplification stages 212, 214, and215. Each of amplifier stages 212-215 includes a distributed gain medium220, 221, 223, respectively, which operate to receive multiplewavelength optical signal 216 and pump wavelengths 224 a-224 c frompumps 222 a-222 c. Each amplifier stage includes an optical coupleroperable to introduce pump wavelengths 224 to the respective gain media.In some embodiments, lossy elements 226 may reside between one or moreamplification stages 212-215. Lossy elements 226 may comprise, forexample, optical add/drop multiplexers, isolators, and/or gainequalizers.

Amplifier 200 may comprise a discrete Raman amplifier or a hybrid Ramanamplifier. For example, first distributed gain medium 220 may comprise atransmission fiber, a section of transmission fiber, or a Raman gainfiber. In a particular embodiment, first distributed gain medium 220could comprise a dispersion compensating Raman gain fiber.

Distributed gain medium 221 of second stage 214 may comprise a segmentof transmission fiber or a Raman gain fiber. Distributed gain medium 223of third amplifier phase 215 could comprise, for example, a Raman gainfiber. In particular embodiments, any or all of distributed gain mediums220-223 could comprise a dispersion compensating Raman gain fiber.

In operation, amplifier 200 receives signal 216 at first stage 212 andapplies a gain to signal wavelengths 216 according to gain profile 230depicted in FIG. 5b. Signal 216 next traverses second stage 214 wheregain profile 240 is applied. Finally, signal 216 is amplified by thirdstage 215 according to gain profile 245 shown in FIG. 3b. Signal 216exits amplifier 200 at output 260 having been exposed to a compositegain profile 250 as shown in FIG. 3c.

In this particular example, first stage 212 and second stage 214 operatein a similar manner to amplifier 100 shown in FIG. 3a. In particular,first stage 212 applies a gain profile 230 that amplifies a majority ofshorter signal wavelengths 216 more than it amplifies a majority oflonger signal wavelengths 216. Second stage 214, conversely, applies andapproximately complimentary gain profile 240 to signal 216, where themajority of longer wavelengths of signal 216 are amplified more than amajority of shorter wavelengths of signal 216.

The combination of second stage 214 and third stage 215, on the otherhand, operates similarly to amplifier 110 shown in FIG. 4. While secondstage 214 applies gain profile 240 amplifying a majority of longersignal wavelengths 216 more than a majority of shorter signalwavelengths 216, third stage 215 applies to gain profile 245, whichamplifies a majority of shorter signal wavelengths 216 more than amajority of longer signal wavelengths 216. In this particular example,gain profile 240 of second stage 214 is approximately complimentary toboth gain profile 230 of first stage 212 and gain profile 245 of thirdstage 215. In this example, the slope of gain profile 240 issignificantly steeper than the slope of gain profiles 230 and 245 toaccount for the fact that gain profile 240 is the only profileexhibiting an upward slope. The composite gain profile 250 (shown inFIG. 5c) resulting from the combination of amplifications in first,second, and third amplifier stages of amplifier 200 results in anapproximately flat gain profile.

This particular example reaps the efficiency benefits discussed withrespect to FIG. 4, and permits use of the noise figure reductiontechniques discussed with respect to FIGS. 3 and 4. For example,efficiency advantages are realized by allowing longer pump wavelengths224 of second stage 214 to accept power from high powered shorter pumpwavelengths 224 c of third amplification stage 215. This results fromthe Raman effect wherein longer wavelength signals tend to rob energyfrom shorter wavelength signals. As a result, second stage 214 can beoperated with fewer wavelength pumps than what otherwise be required,and also with lower pump launch powers.

In terms of improvements in noise figure, the gain profiles of firststage 212 compared to second stage 214 results in high amplification ofshorter wavelengths of signal 216 to overcome phonon stimulated noiseassociated with interaction of those signals with the longer pumpwavelengths 224 a. In addition, providing a significant amount ofamplification to shorter wavelengths of signal 216 in the last stage 215of amplifier 220 helps to minimize the noise figure associated withamplifier 200.

FIGS. 6a-6 c show a block diagram of a four stage Raman amplifier, gainprofiles associated with various stages of the amplifier, and acomposite gain of the amplifier respectively. Amplifier 300 is similarin structure and function to amplifiers 100 and 110 shown in FIGS. 1 and2, respectively. In this example, amplifier 300 includes four Ramanamplification stages 312, 314, 315, and 317. Each amplification stageincludes a distributed gain medium 320, 321, 323, and 325, respectively.Distributed gain medium 320 of first stage 312 may comprise, forexample, a transmission fiber or a Raman gain fiber. Each of distributedgain medium 312-325 of second, third, and fourth stages 314-317 maycomprise a Raman gain fiber or a segment of transmission fiber. Inparticular embodiments, some or all of distributed gain media 320-325could comprise dispersion compensating Raman gain fibers.

Each distributed gain medium 320-325 is operable to receive a multiwavelength optical signal 316 and amplify that signal by facilitatinginteraction between optical signal 316 and pump wavelengths 324 a-324 d.Pump wavelengths 324 are generated by pumps 322 and coupled todistributed gain media 320-325 through couplers 318. In this particularexample, couplers 318 comprise wave division multiplexers.

In the illustrated embodiment, amplifier 300 includes at least one lossyelement 326 coupled between amplifier stages. In this example, lossyelement 326 b comprises an optical add/drop multiplexer coupled betweensecond stage 314 and third stage 315. Optical add/drop multiplexer 326 bfacilitates mid-stage access to amplifier 300 and allows selectiveaddition and/or deletion of particular wavelengths from signal 316.Other lossy elements, such as isolators or gain equalizers couldalternatively reside between amplifier stages.

In operation, signal 316 enters amplifier 300 at coupler 318 a, whichpasses signal 316 to first amplifier stage 312 where a gain profile at330, as shown in FIG. 4b, is applied to wavelengths of signal 316.Signal 316 is then passed to second stage 314 where a gain profile 335,as shown in FIG. 4b is applied to wavelengths of signal 316.

In this particular example, first and second stages 312 and 314 ofamplifier 300 operate similarly to amplifier 100 described with respectto FIG. 3. In particular, first stage 312 applies a gain profile where amajority of shorter signal wavelengths are amplified more than amajority of longer signal wavelengths, and second stage 314 applies anapproximately complimentary gain profile 335 where a majority of longersignal wavelengths are amplified more than a majority of shorter signalwavelengths. In this particular embodiment, the composite gain fromfirst stage 312 and second stage 314 results in an approximately flatgain profile at the output of second stage 314. This designadvantageously facilitates addition and subtraction of particularwavelengths of signal 316 without the need for further manipulation ofthe gain. In addition, first and second gain stages 312 and 314 providea low noise. figure, reducing the effects of phonon stimulated noise inshorter wavelength signals closest to the pump wavelengths.

Continuing with the operational description, particular wavelengths ofsignal 316 may be substituted with other wavelengths at add/dropmultiplexer 326 b. After processing by add/drop multiplexer 326 b,signal 316 continues to third amplification stage 315, where gainprofile 340 is applied as shown in FIG. 6b. Signal 316 is thencommunicated to fourth stage 317 where gain profile 345 is applied towavelengths of signal 316. Amplified signal 316 is then output at outputport 365.

Third and fourth amplification stages of amplifier 300 are similar instructure and function to amplifier 110 described with respect to FIG.4. Through the use of this configuration, third and fourth amplifierstages 315 and 317 provide increased efficiency in operation. Inparticular, pump 322 can operate with fewer pump signals and/or lowerlaunch power as a result of the Raman scattering effect which allowslonger pump wavelengths 324 c of third stage 316 to accept power fromhighly amplified shorter pump wavelengths 324 d of fourth stage 317.Moreover, third and fourth amplification stages 315 and 317 assist inmaintaining a low noise figure by applying a significant amount of thegain to the shortest wavelengths of signal 316 at the last amplifierstage 317.

FIG. 7 is a flow chart showing one example of a method 400 of amplifyinga multi-wavelength optical signal using a multi-stage Raman amplifier.This particular example uses FIGS. 6a-6 c to illustrate the method.Similar methods could apply to any of the embodiments described herein.Method 400 begins at step 410 where first amplifier stage 312 receivessignal wavelengths 316 and applies first gain profile 330 to thosewavelengths. Step 420 allows for optional mid-stage access between firststage 312 and second stage 314. The method continues where second stage314 applies second gain profile 325 to signal wavelengths 316 at step430.

Second gain profile 335 is approximately complimentary to first gainprofile 330. In this particular example, first gain profile 330amplifies a majority of shorter signal wavelengths 316 more than amajority of longer signal wavelengths 316, while second gain profile 325amplifies a majority of longer wavelength signals 316 more than amajority of shorter wavelength signals 316. Those gain profiles could bereversed if desired. Moreover, additional gain profiles could be appliedbetween first stage 312 and second stage 314 by intervening stages (notexplicitly shown). This particular example shows additional stagesbeyond first stage 312 and second stage 314. In a particular embodiment,an amplifier embodying the invention could comprise only twocomplimentary stages of Raman gain.

This example provides optional mid-stage access at step 450. Mid-stageaccess could comprise, for example, application of optical add/dropmultiplexing, gain equalization, or the presence of one or more opticalisolators.

Where amplifier 300 comprises more than two stages of complimentaryRaman amplification, method 400 continues at step 460 where third stage316 applies gain profile 340 to signal wavelengths 316. Where amplifier300 comprises a three stage amplifier, third gain profile 340 can becomplimentary to second gain profile 335. An example of this operationis shown in FIG. 5. Where amplifier 300 comprises a four stageamplifier, third stage 315 can apply gain profile at 340 as shown inFIG. 6b, while fourth stage 317 applies gain profile 345 as shown inFIG. 6b at step 480.

In this example, third gain profile 340 amplifies a majority of longersignal wavelengths 316 more than a majority of shorter signalwavelengths 316 while fourth stage 317 amplifies a majority of shortersignal wavelengths 316 more than a majority of longer signal wavelengths316. In this manner, third and fourth stages of amplifier 300 canrealize efficiency advantages by allowing longer pump wavelengths 324 cfrom third stage 315 to accept energy from highly amplified shorter pumpwavelengths 324 d in fourth stage 317.

Although this method has described a four stage amplification process,the method can equally apply to any system having two or more Ramanamplification stages. In addition, although this particular exampledescribed first and second gain stages having gain profiles 330 and 335as shown in FIG. 6b, and third and fourth gain stages having gainprofiles 340 and 345 as shown in FIG. 6b, those gain profiles could bereversed without departing from the scope of the invention. Theparticular example shown provides significant advantages in a four stageamplifier in that initial stages can be configured to provide a lownoise figure by emphasizing amplification of shorter wavelength signalsearly in the amplification process. In addition, third and fourthamplification stages advantageously realize efficiency gains inamplifier locations where noise reduction is not as critical a concern.

FIGS. 8a-8 b are graphs showing simulations of one aspect of the presentinvention embodied in a two stage distributed Raman amplifier. FIGS.9a-9 b are graphs showing simulations of one aspect of the presentinvention embodied in a two stage discrete Raman amplifier. Theparameters used for the amplifier simulations were as follows:

Distributed Discrete Stage 1 Input Port Loss 0 dB 1.3 dB Stage 1 GainFiber 80 km LEAF fiber DK-21 (DCF) Stage 1 Pump Powers: 438 mW @ 1396 nm438 mW @ 1416 nm 380 mW @ 416 nm 438 mW @ 1427 nm 380 mW @ 1427 nm 170mW @ 1450 nm 220 mW @ 1450 nm 10 mW @ 1472 nm 4 mW @ 1505 nm 19 mW @1505 nm Mid-Stage Loss 2 dB 1.6 dB Stage 2 Gain Fiber DK-30 (DCF) DK-19(DCF) Stage 2 Pump Powers: 380 mW @ 1399 nm 380 mW @ 1472 nm 380 mW @1472 nm 380 mW @ 1505 nm 380 mW @ 1505 nm Stage 2 Output Port 1 dB 1.3dB Loss

FIGS. 8a and 9A show first gain profile 30 of first stage 112, secondgain profile 40 of second stage 114, and composite gain profile 50 ofRaman amplifier 100 for distributed and discrete configurations,respectively. As shown in these figures application of pump wavelengths124 as shown in Table 1 above results in a downwardly sloping gainprofile 30 for first stage 112, and an upwardly sloping gain profile 40for second stage 114. Gain profiles 30 and 40 are approximatelycomplementary to one another, although they do not comprise mirrorimages of one another.

The composite gain profile 50 of amplifier 100 is approximately flatacross the bandwidth of signal 116 being amplified. Gain profile 50represents the gain profile without application of any gain flatteningfilters. In this embodiment, amplifier 100 obtains an overall gainprofile that is approximately flat for over 100 nanometers.

FIGS. 8b and 9 b show the same gain profile 50 and compare that profileto the noise figure of the amplifier. In the case of the discrete Ramanamplifier simulated in FIG. 9b, the actual noise FIG. 55 is shown. Inthe case of the distributed Raman amplifier simulated in FIG. 8b, theeffective noise FIG. 65 is shown.

An optical amplifier noise figure is defined as NF=SNRin/SNRout whereSNRin is the signal-to-noise ratio of the amplifier input signal andSNRout is the signal-to-noise ratio of the amplifier output signal. Asdefined, NF is always greater than 1 for any realizable amplifier.Effective noise figure for a distributed optical amplifier is defined asthe noise figure a discrete amplifier placed at the end of thedistributed amplifier transmission fiber would need to have to producethe same final SNR as the distributed amplifier. It can be, and inpractice is, less than 1 (negative value in dB) for practicaldistributed amplifiers over at least a small portion of their operatingwavelength range.

As shown in FIGS. 8b and 9 b, the noise figure in this embodiment isalways less than eight decibels over the entire bandwidth of signal 116.In fact, for a bandwidth between 1520 nanometers and 1620 nanometers,the noise figure never exceeds 7 decibels.

Although the present invention has been described in severalembodiments, a myriad of changes, variations, alterations,transformations, and modifications may be suggested to one skilled inthe art, and it is intended that the present invention encompass suchchanges, variations, alterations, transformations, and modifications asfall within the spirit and scope of the appended claims.

What is claimed is:
 1. A multi-stage optical amplifier, comprising: a first Raman amplifier stage having a first sloped gain profile operable to amplify a plurality of signal wavelengths comprising a bandwidth of at least sixty (60) nanometers; a second Raman amplifier stage having a second sloped gain profile operable to amplify at least most of the plurality of signal wavelengths after those wavelengths have been amplified by the first stage, the second sloped gain profile having an approximately complementary slope to the slope of the first sloped gain profile; wherein the combined effect of the first and second Raman stages contributes to an approximately flat overall gain profile over the plurality of signal wavelengths.
 2. The amplifier of claim 1, wherein the first and second Raman stages operate to amplify all of the same signal wavelengths.
 3. The amplifier of claim 1, wherein the slope of the first gain profile has an approximately equal and opposite slope from the slope of the second gain profile.
 4. The amplifier of claim 1, wherein: the first sloped gain profile comprises a gain profile wherein a majority of shorter signal wavelengths are amplified more than a majority of longer signal wavelengths; and the second sloped gain profile comprises a gain profile wherein a majority of the longer signal wavelengths are amplified more than a majority of the shorter signal wavelengths.
 5. The amplifier of claim 4, wherein a phonon stimulated noise figure of the amplifier comprises less than four decibels.
 6. The amplifier of claim 4, wherein a small signal noise figure of the amplifier comprises less than eight decibels.
 7. The amplifier of claim 4, wherein a small signal noise figure of the amplifier comprises less than seven decibels.
 8. The amplifier of claim 4, further comprising a third Raman amplifier stage having a third sloped gain profile comprising a gain profile wherein a majority of shorter signal wavelengths are amplified more than a majority of longer signal wavelengths, the third Raman stage operable to amplify approximately the same plurality of signal wavelengths after those wavelengths have been amplified by the second Raman stage.
 9. The amplifier of claim 8, wherein the slope of the second gain profile is opposite from and steeper than the slope of the first or the third gain profiles.
 10. The amplifier of claim 8, wherein the combined effect of the first, second, and third, Raman stages contributes to an approximately flat overall gain profile over the plurality of signal wavelengths.
 11. The amplifier of claim 4, further comprising: a third Raman amplifier stage having a third sloped gain profile wherein a majority of longer signal wavelengths are amplified more than a majority of shorter signal wavelengths, the third Raman stage operable to amplify approximately the same plurality of signal wavelengths after those wavelengths are amplified by the second Raman stage; and a fourth Raman amplifier stage having a fourth sloped gain profile wherein a majority of shorter signal wavelengths are amplified more than a majority of longer signal wavelengths; the fourth Raman stage operable to amplify approximately the same plurality of signal wavelengths after those wavelengths are amplified by the third Raman stage.
 12. The amplifier of claim 11, further comprising a lossy element coupled between the second and third Raman stages, the lossy element selected from a group consisting of a gain equalizer and an optical isolator.
 13. The amplifier of claim 11, further comprising a lossy element coupled between the second and third Raman stages and operable to provide mid-stage access to the amplifier.
 14. The amplifier of claim 13, wherein the lossy element comprises an optical add/drop multiplexer.
 15. The amplifier of claim 13, wherein the combined effect of the first, second, third, and fourth Raman stages contributes to an approximately flat overall gain profile over the plurality of signal wavelengths.
 16. The amplifier of claim 12, wherein: the combined effect of the first and second Raman stages contributes to an approximately flat overall gain profile over the plurality of signal wavelengths output from the second stage; and wherein the combined effect of the third and fourth Raman stages contributes to an approximately flat overall gain profile over the plurality of signal wavelengths output from the fourth stage.
 17. The amplifier of claim 1, wherein: the first sloped gain profile comprises a gain profile wherein a majority of longer signal wavelengths are amplified more than a majority of shorter signal wavelengths; and the second sloped gain profile comprises a gain profile wherein a majority of the shorter signal wavelengths are amplified more than a majority of the longer signal wavelengths.
 18. The amplifier of claim 17, wherein the first Raman stage is coupled to the second Raman stage so as to allow longer pump wavelengths in the first Raman stage to accept power from shorter pump wavelengths in the second Raman stage.
 19. The amplifier of claim 1, wherein the amplifier comprises a discrete Raman amplifier.
 20. The amplifier of claim 1, wherein the amplifier comprises a distributed Raman amplifier.
 21. The amplifier of claim 1, wherein the amplifier comprises a hybrid Raman amplifier.
 22. The amplifier of claim 1, wherein each of the Raman amplifier stages comprises a plurality of pump wavelength signals collectively operable to affect the slope and magnitude of the gain profile for that stage.
 23. The amplifier of claim 22, wherein the longest pump wavelength signal comprises a wavelength at least ten nanometers shorter than the shortest wavelength of the plurality of signal wavelengths.
 24. The amplifier of claim 22, wherein a majority of the gain applied to signal wavelengths within thirty nanometers of the longest pump wavelength signal is applied in the first Raman stage of the amplifier.
 25. The amplifier of claim 22, wherein a majority of the gain applied to signal wavelengths within forty five nanometers of the longest pump wavelength signal is applied in the first Raman stage of the amplifier.
 26. The amplifier of claim 22, wherein a majority of the gain supplied by the longest pump wavelength signal is applied in a last Raman stage of the amplifier.
 27. The amplifier of claim 1, wherein the bandwidth of the plurality of signal wavelengths comprises more than eighty nanometers.
 28. The amplifier of claim 1, wherein the bandwidth of the plurality of signal wavelengths comprises at least one hundred nanometers.
 29. The amplifier of claim 1, wherein the overall gain profile of the amplifier prior to use of a gain flattening filter varies by less than five decibels within the bandwidth of the plurality of signal wavelengths.
 30. The amplifier of claim 1, further comprising a gain flattening filter coupled to the amplifier, the gain flattening filter operable to further flatten the gain profile of the amplifier.
 31. The amplifier of claim 1, further comprising a lossy element coupled between at least two of the Raman stages.
 32. The amplifier of claim 1, wherein the plurality of wavelength signals comprises a bandwidth of at least sixty-five (65) nanometers.
 33. The amplifier of claim 1, wherein the plurality of wavelength signals comprises a bandwidth of at least seventy (70) nanometers.
 34. The amplifier of claim 1, wherein the plurality of wavelength signals comprises a bandwidth of at least seventy five (75) nanometers.
 35. The amplifier of claim 1, wherein the overall gain profile of the amplifier prior to use of a gain flattening filter varies by less than four (4) decibels over the bandwidth of the plurality of signal wavelengths.
 36. The amplifier of claim 1, wherein the overall gain profile of the amplifier prior to use of a gain flattening filter varies by less than three (3) decibels over the bandwidth of the plurality of signal wavelengths.
 37. The amplifier of claim 1, wherein each stage of the amplifier comprises: an input operable to receive an optical signal; an output operable to communicate an amplified version of the optical signal; a distributed gain medium for receiving the optical signal and amplifying the optical signal therein through nonlinear interaction; a pump operable to generate a pump wavelength; and a coupler operable to deliver the pump wavelength to the distributed gain medium.
 38. The amplifier of claim 37, wherein the distributed gain medium comprises a transmission fiber.
 39. The amplifier of claim 37, wherein the distributed gain medium comprises a Raman gain fiber.
 40. The amplifier of claim 37, wherein the distributed gain medium comprises a dispersion compensating Raman gain fiber.
 41. The amplifier of claim 37, wherein the pump comprises one or more laser diodes.
 42. The amplifier of claim 37, wherein the coupler comprises a wavelength division multiplexer.
 43. The amplifier of claim 1, further comprising at least one additional amplification stage coupled between the first and second Raman amplification stages.
 44. A method of amplifying an optical signal having multiple wavelengths, the method comprising: amplifying a plurality of signal wavelengths comprising a bandwidth of at least sixty (60) nanometers at a first Raman amplifier stage having a first sloped gain profile; amplifying at least most of the plurality of signal wavelengths at a second Raman amplifier stage after those signal wavelengths have been amplified by the first stage, the second stage having a second sloped gain profile comprising an approximately complimentary gain profile to the first gain profile; wherein the combined effect of the first and second Raman stages contributes to an approximately flat overall gain profile over the plurality of signal wavelengths.
 45. The method of claim 44, wherein the first and second Raman stages operate to amplify all of the same signal wavelengths.
 46. The method of claim 44, wherein the slope of the first gain profile has an approximately equal and opposite slope from the slope of the second gain profile.
 47. The method of claim 44, wherein: the first sloped gain profile comprises a gain profile wherein a majority of shorter signal wavelengths are amplified more than a majority of longer signal wavelengths; and the second sloped gain profile comprises a gain profile wherein a majority of the longer signal wavelengths are amplified more than a majority of the shorter signal wavelengths.
 48. The method of claim 47, wherein a small signal noise figure of the amplifier comprises less than eight decibels.
 49. The method of claim 35, further comprising amplifying at least most of the plurality of signal wavelengths at a third Raman amplifier stage after those signal wavelengths have been amplified by the second stage, the third stage having a third sloped gain profile comprising an approximately complimentary gain profile to the second gain profile.
 50. The method of claim 49, wherein the first sloped gain profile and the third gain profile comprise different slopes.
 51. The method of claim 47, further comprising: amplifying at least most of the plurality of signal wavelengths at a third Raman amplifier stage after those signal wavelengths have been amplified by the second stage, the third stage having a third sloped gain profile wherein a majority of longer signal wavelengths are amplified more than a majority of shorter signal wavelengths; and amplifying at least most of the plurality of signal wavelengths at a fourth Raman amplifier stage after those signal wavelengths have been amplified by the third stage, the fourth stage having a fourth sloped gain profile wherein a majority of shorter signal wavelengths are amplified more than a majority of shorter signal wavelengths.
 52. The method of claim 44, wherein: the first sloped gain profile comprises a gain profile wherein a majority of longer signal wavelengths are amplified more than a majority of shorter signal wavelengths; and the second sloped gain profile comprises a gain profile wherein a majority of the shorter signal wavelengths are amplified more than a majority of the longer signal wavelengths.
 53. The method of claim 52, wherein the first Raman stage is coupled to the second Raman stage so as to allow longer pump wavelengths in the first Raman stage to accept power from shorter pump wavelengths in the second Raman stage.
 54. The method of claim 44, wherein each of the Raman amplifier stages comprises a plurality of pump wavelength signals collectively operable to affect the slope and magnitude of the gain profile for that stage.
 55. The method of claim 54, wherein the longest pump wavelength signal comprises a wavelength at least ten nanometers shorter than the shortest wavelength of the plurality of signal wavelengths.
 56. The method of claim 54, wherein a majority of the gain supplied by the longest pump wavelength signal is applied in a last Raman stage of the amplifier.
 57. The method of claim 44, wherein the bandwidth of the plurality of signal wavelengths comprises more than eighty nanometers.
 58. The method of claim 44, wherein the overall gain profile of the amplifier prior to use of a gain flattening filter varies by less than five decibels within the bandwidth of the plurality of signal wavelengths.
 59. The method of claim 44, further comprising amplifying the plurality of wavelengths between the first and second Raman amplification stages.
 60. A multi-stage optical amplifier, comprising: a plurality of cascaded Raman amplifier stages each having a gain profile, wherein the gain profile of at least some of the Raman stages is sloped; wherein at least two of the sloped gain profiles comprise approximately complimentary gain profiles, and wherein the combined effect of the gain profiles of the Raman stages contributes to an approximately flat overall gain profile over a plurality of signal wavelengths amplified by the amplifier, wherein the plurality of signal wavelengths comprise a bandwidth of at least sixty (60) nanometers.
 61. The amplifier of claim 60, wherein the first and second Raman stages operate to amplify all of the same signal wavelengths.
 62. The amplifier of claim 60, wherein the slope of the first gain profile has an approximately equal and opposite slope from the slope of the second gain profile.
 63. The amplifier of claim 60, wherein: the first sloped gain profile comprises a gain profile wherein a majority of shorter signal wavelengths are amplified more than a majority of longer signal wavelengths; and the second sloped gain profile comprises a gain profile wherein a majority of the longer signal wavelengths are amplified more than a majority of the shorter signal wavelengths.
 64. The amplifier of claim 63, further comprising a third Raman amplifier stage having a third sloped gain profile comprising a gain profile wherein a majority of shorter signal wavelengths are amplified more than a majority of longer signal wavelengths, the third Raman stage operable to amplify approximately the same plurality of signal wavelengths after those wavelengths have been amplified by the second Raman stage.
 65. The amplifier of claim 64, wherein the combined effect of the first, second, and third, Raman stages contributes to an approximately flat overall gain profile over the plurality of signal wavelengths.
 66. The amplifier of claim 63, further comprising: a third Raman amplifier stage having a third sloped gain profile wherein a majority of longer signal wavelengths are amplified more than a majority of shorter signal wavelengths, the third Raman stage operable to amplify approximately the same plurality of signal wavelengths after those wavelengths are amplified by the second Raman stage; and a fourth Raman amplifier stage having a fourth sloped gain profile wherein a majority of shorter signal wavelengths are amplified more than a majority of shorter signal wavelengths; the fourth Raman stage operable to amplify approximately the same plurality of signal wavelengths after those wavelengths are amplified by the third Raman stage.
 67. The amplifier of claim 66, wherein the combined effect of the first, second, third, and fourth Raman stages contributes to an approximately flat overall gain profile over the plurality of signal wavelengths.
 68. The amplifier of claim 66, wherein: the combined effect of the first and second Raman stages contributes to an approximately flat overall gain profile over the plurality of signal wavelengths output from the second stage; and wherein the combined effect of the third and fourth Raman stages contributes to an approximately flat overall gain profile over the plurality of signal wavelengths output from the fourth stage.
 69. The amplifier of claim 60, wherein: the first sloped gain profile comprises a gain profile wherein a majority of longer signal wavelengths are amplified more than a majority of shorter signal wavelengths; and the second sloped gain profile comprises a gain profile wherein a majority of the shorter signal wavelengths are amplified more than a majority of the longer signal wavelengths.
 70. The amplifier of claim 69, wherein the first Raman stage is coupled to the second Raman stage so as to allow longer pump wavelengths in the first Raman stage to accept power from shorter pump wavelengths in the second Raman stage.
 71. The amplifier of claim 60, wherein each of the Raman amplifier stages comprises a plurality of pump wavelength signals collectively operable to affect the slope and magnitude of the gain profile for that stage.
 72. The amplifier of claim 71, wherein the longest pump wavelength signal comprises a wavelength at least ten nanometers shorter than the shortest wavelength of the plurality of signal wavelengths.
 73. The amplifier of claim 71, wherein a majority of the gain supplied by the longest pump wavelength signal is applied in a last Raman stage of the amplifier.
 74. The amplifier of claim 60, wherein the bandwidth of the plurality of signal wavelengths comprises more than eighty nanometers.
 75. The amplifier of claim 60, wherein the overall gain profile of the amplifier prior to use of a gain flattening filter varies by less than five decibels within the bandwidth of the plurality of signal wavelengths.
 76. The amplifier of claim 60, further comprising at least one additional amplification stage coupled between the first and second stages.
 77. A method of amplifying multiple-wavelength optical signals, comprising: applying a first sloped gain profile to a plurality of signal wavelengths at a first stage of a Raman amplifier, wherein the plurality of signal wavelengths comprise a bandwidth of at least sixty (60) nanometers; applying a second sloped gain profile to at least most of the plurality of signal wavelengths at a second stage of the Raman amplifier, the second gain profile comprising an approximately complementary gain profile of the first sloped gain profile; wherein the combined effect of the first and second sloped gain profiles contributes to an approximately flat overall gain profile over the plurality of signal wavelengths.
 78. A multi-stage optical amplifier, comprising: a plurality of cascaded Raman amplifier stages each operable to amplify a plurality of signal wavelengths and each having a gain profile determined at least in part by one or more pump wavelength signals applied to that amplifier stage; wherein the cascaded Raman amplifier stages comprise at least one Raman amplifier stage having an input coupled to an output of another of the plurality of cascaded Raman amplifier stages, and wherein the plurality of amplifier stages comprise a first Raman stage operable to apply a higher gain level to a signal wavelength closest to a longest pump wavelength than a gain applied to a signal wavelength furthest from the longest pump wavelength.
 79. The amplifier of claim 78, wherein the highest level of gain supplied by the longest pump wavelength signal is supplied in a last Raman stage of the amplifier.
 80. The multi-stage Raman amplifier of claim 78, wherein at least one of the plurality of cascaded Raman amplifier stages receives at least one pump wavelength signal that is not received by at least one other of the plurality of cascaded Raman amplifier stages.
 81. A method of amplifying an optical signal having multiple wavelengths, the method comprising: receiving a plurality of signal wavelengths at a plurality of cascaded Raman amplifier stages having at least a first Raman amplification stage and a last Raman amplification stage, each Raman amplification stage operable to amplify a plurality of signal wavelengths and each Raman amplification stage having a gain profile determined at least in part by one or more pump wavelength signals applied to the amplifier stage; applying a highest level of gain supplied by the longest pump wavelength in the last Raman amplification stage of the amplifier.
 82. A multi-stage optical amplifier, comprising: a plurality of cascaded Raman amplifier stages, at least some of the Raman stages having sloped gain profiles operable to contribute to a combined gain profile of the amplifier; wherein the combined gain profile of the amplifier is approximately flat across a bandwidth of at least sixty (60) nanometers and comprises a small signal noise figure no greater than six (6) decibels.
 83. The amplifier of claim 82, wherein the at least some of the Raman stages operate to amplify all of the same signal wavelengths.
 84. The amplifier of claim 82, wherein at least two of the plurality of cascaded Raman gain stages comprise approximately complementary gain profiles.
 85. The amplifier of claim 82, wherein the plurality of cascaded Raman gain stages comprise: a first sloped gain profile wherein a majority of shorter signal wavelengths are amplified more than a majority of longer signal wavelengths; and a second sloped gain profile wherein a majority of the longer signal wavelengths are amplified more than a majority of the shorter signal wavelengths.
 86. The amplifier of claim 82, wherein the plurality of cascaded Raman gain stages comprise: a first sloped gain profile wherein a majority of longer signal wavelengths are amplified more than a majority of shorter signal wavelengths; and a second sloped gain profile wherein a majority of the shorter signal wavelengths are amplified more than a majority of the longer signal wavelengths.
 87. The amplifier of claim 86, wherein the first Raman stage is coupled to the second Raman stage so as to allow longer pump wavelengths in the first Raman stage to accept power from shorter pump wavelengths in the second Raman stage.
 88. The amplifier of claim 82, wherein the bandwidth of the plurality of signal wavelengths comprises at least one hundred nanometers.
 89. The amplifier of claim 82, wherein the overall gain profile of the amplifier prior to use of a gain flattening filter varies by less than five decibels within the bandwidth of the plurality of signal wavelengths.
 90. The amplifier of claim 82, further comprising at least one additional amplification stage coupled between the first and second stages.
 91. The amplifier of claim 82, wherein the small signal noise figure is no greater than seven decibels.
 92. A method of amplifying an optical signal having multiple wavelengths, the method comprising: amplifying a plurality of signal wavelengths at a first Raman amplifier stage having a first sloped gain profile; amplifying at least most of the plurality of signal wavelengths at a second Raman amplifier stage having a second sloped gain profile that is different than the first sloped gain profile; wherein the combined gain profile of the amplifier is approximately flat across a bandwidth of at least sixty (60) nanometers and comprises a small signal noise figure no greater than six (6) decibels.
 93. The method of claim 92, wherein the small signal noise figure is no greater than seven decibels.
 94. An optical pre-amplifier operable to be coupled to an optical communication link carrying optical signals having a plurality of wavelengths, the preamplifier comprising: a first Raman stage having a gain profile where a majority of shorter signal wavelengths are amplified more than a majority of longer signal wavelengths; and a second Raman stage operable to receive at least most of the signal wavelengths after they have been amplified by the first stage, the second stage having a gain profile where a majority of longer signal wavelengths are amplified more than a majority of shorter signal wavelengths; wherein the gain profiles of the first and second Raman stages are operable to combine to contribute to an approximately flat combined gain profile over the plurality of signal wavelengths.
 95. The preamplifier of claim 94, wherein the small signal noise figure of the amplifier is less than eight decibels.
 96. The preamplifier of claim 94, wherein the bandwidth of the plurality of signal wavelengths comprises at least eighty nanometers.
 97. The preamplifier of claim 94, wherein the overall gain profile of the amplifier prior to use of a gain flattening filter varies by less than five decibels within the bandwidth of the plurality of signal wavelengths.
 98. The preamplifier of claim 94, further comprising at least one additional amplification stage coupled between the first and second stages.
 99. The booster amplifier of claim 94, wherein the bandwidth of the plurality of signal wavelengths comprises at least eighty nanometers.
 100. The booster amplifier of claim 94, wherein the overall gain profile of the amplifier prior to use of a gain flattening filter varies by less than five decibels within the bandwidth of the plurality of signal wavelengths.
 101. The booster amplifier of claim 94, further comprising at least one additional amplification stage coupled between the first and second stages.
 102. The booster amplifier of claim 94, wherein the first Raman stage is coupled to the second Raman stage so as to allow longer pump wavelengths in the first Raman stage to accept power from shorter pump wavelengths in the second Raman stage.
 103. The booster amplifier of claim 94, wherein the booster amplifier implements no more than eight pump wavelengths per stage.
 104. An optical booster amplifier operable to be coupled to an optical communication link carrying optical signals having a plurality of wavelengths, wherein the plurality of signal wavelengths comprise a bandwidth of at least sixty (60) nanometers, the booster amplifier comprising: a first Raman stage having a gain profile where a majority of longer signal wavelengths are amplified more than a majority of shorter signal wavelengths; and a second Raman stage operable to receive at least most of the signal wavelengths after they have been amplified by the first stage, the second stage having a gain profile where a majority of shorter signal wavelengths are amplified more than a majority of longer signal wavelengths; wherein the gain profiles of the first and second Raman stages are operable to combine to contribute to an approximately flat combined gain profile over the plurality of wavelengths.
 105. The booster amplifier of claim 104, wherein the small signal noise figure of the amplifier is less than eight decibels.
 106. A Raman amplifier assembly comprising: a preamplifier coupled to an optical communication link and comprising: a first Raman stage having a gain profile wherein a majority of shorter wavelengths are amplified more than a majority of longer wavelengths; and a second Raman stage having a gain profile approximately complementary to the first gain stage; and a booster amplifier coupled to the optical communication link and comprising: a first Raman stage having a gain profile wherein a majority of longer wavelengths are amplified more than a majority of shorter wavelengths; and a second Raman stage having a gain profile approximately complementary to the first gain stage.
 107. An optical communication system operable to facilitate communication of multiple signal wavelengths, the system comprising: a transmitter bank operable to generate a plurality of signal wavelengths; a multiplexer operable to combine the plurality of signal wavelengths into a single multiple wavelength signal for transmission over a transmission medium; an amplifier coupled to the transmission medium and operable to amplify the multiple wavelength signal prior to, during, or after the multiple wavelength signal's transmission over the transmission medium, the amplifier comprising a multi-stage Raman amplifier, comprising: a first Raman amplifier stage having a first sloped gain profile operable to amplify a plurality of signal wavelengths comprising a bandwidth of at least sixty (60) nanometers; and a second Raman amplifier stage having a second sloped gain profile operable to amplify at least most of the plurality of signal wavelengths after those wavelengths have been amplified by the first stage, the second sloped gain profile having an approximately complementary slope to the slope of the first sloped gain profile; wherein the combined effect of the first and second Raman stages contributes to an approximately flat overall gain profile over the plurality of signal wavelengths; a demultiplexer operable to receive the multiple wavelength signal and to separate the signal wavelengths from the multiple wavelength signal; and a receiver bank operable to receive the plurality of signal wavelengths.
 108. A multi-stage optical amplifier, comprising: a first Raman amplifier stage operable to amplify a plurality of signal wavelengths and having a first sloped gain profile wherein a majority of shorter signal wavelengths are amplified more than a majority of longer signal wavelengths; a second Raman amplifier stage operable to amplify at least most of the plurality of signal wavelengths and having a second sloped gain profile wherein a majority of longer signal wavelengths are amplified more than a majority of shorter signal wavelengths; and a third Raman amplifier stage having a third sloped gain profile wherein a majority of shorter signal wavelengths are amplified more than a majority of longer signal wavelengths, the third Raman stage operable to amplify approximately the same plurality of signal wavelengths after those wavelengths have been amplified by the second Raman stage; wherein the combined effect of the first, second, and third amplifier stages contributes to an approximately flat overall gain profile over the plurality of signal wavelengths.
 109. The amplifier of claim 108 wherein the slope of the second gain profile is opposite from and steeper than the slope of the first or the third gain profiles.
 110. The amplifier of claim 108, wherein each of the amplifier stages operates to amplify all of the same signal wavelengths.
 111. The amplifier of claim 108, wherein at least one of the cascaded amplifier stages comprises a Raman amplifier stage.
 112. The amplifier of claim 108, wherein all of the cascaded amplifier stages comprise Raman amplifier stages.
 113. A multi-stage optical amplifier, comprising: an optical amplifier comprising an odd number of cascaded Raman amplifier stages, each of the odd number of Raman amplifier stages having a sloped gain profile approximately complementary to a gain profile of at least one adjacent amplifier stage and each operable to amplify at least most of a plurality of signal wavelengths; wherein the combined effect of the odd number of cascaded Raman amplifier stages contributes to an approximately flat overall gain profile over the plurality of signal wavelengths.
 114. The amplifier of claim 113, wherein a phonon stimulated noise figure of the amplifier comprises less than four decibels.
 115. The amplifier of claim 113, wherein at least two of the amplifier stages comprise Raman amplifier stages coupled to one another so as to cause pump wavelengths in the one Raman stage to accept power from pump wavelengths in the another Raman stage.
 116. The amplifier of claim 113, wherein the bandwidth of the plurality of signal wavelengths comprises more than sixty (60) nanometers.
 117. The amplifier of claim 113, wherein the overall gain profile of the amplifier prior to use of a gain flattening filter varies by less than five decibels within the bandwidth of the plurality of signal wavelengths.
 118. The amplifier of claim 113, wherein at least one of the cascaded amplifier stages comprises a Raman amplifier stage.
 119. A method of amplifying an optical signal having multiple wavelengths, the method comprising: amplifying a plurality of signal wavelengths at a first Raman amplifier having a first sloped gain profile wherein a majority of shorter signal wavelengths are amplified more than a majority of longer signal wavelengths; amplifying at least most of the plurality of signal wavelengths at a second Raman amplifier stage after those signal wavelengths have been amplified by the first stage, the second stage having a second sloped gain wherein a majority of longer signal wavelengths are amplified more than a majority of shorter signal wavelengths; and amplifying at least most of the plurality of signal wavelengths at a third Raman amplifier stage after those signal wavelengths have been amplified by the second stage, the third stage having a third sloped gain profile comprising an approximately complimentary gain profile to the second gain profile; wherein the combined effect of the first, second, and third amplifier stages contributes to an approximately flat overall gain profile over the plurality of signal wavelengths.
 120. The method of claim 119, wherein the slope of the second gain profile is opposite from and steeper than the slope of the first or the third gain profiles.
 121. The method of claim 119, wherein the bandwidth of the plurality of signal wavelengths comprises more than sixty (60) nanometers.
 122. A method of amplifying an optical signal having multiple wavelengths, the method comprising: amplifying a plurality of signal wavelengths using an optical amplifier comprising an odd number of cascaded Raman amplifier stages, each of the odd number of Raman amplifier stages having a sloped gain profile approximately complementary to a gain profile of at least one adjacent amplifier stage and each operable to amplify at least most of the plurality of signal wavelengths; wherein the combined effect of the odd number of cascaded Raman amplifier stages contributes to an approximately flat overall gain profile over the plurality of signal wavelengths.
 123. The method of claim 122, wherein at least one of the cascaded amplifier stages comprises a Raman amplifier stage.
 124. The method of claim 122, wherein the bandwidth of the plurality of signal wavelengths comprises more than sixty (60) nanometers.
 125. The method of claim 122, wherein the overall gain profile of the amplifier prior to use of a gain flattening filter varies by less than five decibels within the bandwidth of the plurality of signal wavelengths.
 126. A multi-stage optical amplifier, comprising: a plurality of cascaded Raman amplifier stages each having a gain profile, wherein the gain profile of at least some of the Raman stages is sloped; wherein at least two of the sloped gain profiles comprise approximately complimentary gain profiles, and wherein the combined effect of the gain profiles of the Raman stages contributes to an approximately flat overall gain profile over a plurality of signal wavelengths amplified by the amplifier; and wherein at least two of the Raman amplifier stages are coupled so as to cause longer pump wavelengths in one of the at least two Raman stages to accept power from shorter pump wavelengths in another of the at least two Raman stages.
 127. The amplifier of claim 126, wherein the plurality of cascaded Raman amplifier stages comprise: a first Raman amplifier stage having a first sloped gain profile operable to amplify at least most of the plurality of signal wavelengths wherein a majority of longer signal wavelengths are amplified more than a majority of shorter signal wavelengths; and a second Raman amplifier stage operable to amplify at least most of the plurality of signal wavelengths after those wavelengths have been amplified by the first stage, wherein the second Raman stage comprises a second sloped gain profile wherein a majority of the shorter signal wavelengths are amplified more than a majority of the longer signal wavelengths.
 128. The amplifier of claim 126, wherein the plurality of cascaded Raman amplifier stages comprise: a first Raman amplifier stage having a first sloped gain profile operable to amplify at least most of the plurality of signal wavelengths wherein a majority of shorter signal wavelengths are amplified more than a majority of longer signal wavelengths; and a second Raman amplifier stage operable to amplify at least most of the plurality of signal wavelengths after those wavelengths have been amplified by the first stage, wherein the second Raman stage comprises a second sloped gain profile wherein a majority of the longer signal wavelengths are amplified more than a majority of the shorter signal wavelengths.
 129. The amplifier of claim 126, wherein a phonon stimulated noise figure of the amplifier comprises less than four decibels.
 130. The amplifier of claim 126, wherein the bandwidth of the plurality of signal wavelengths comprises more than sixty (60) nanometers.
 131. The amplifier of claim 126, wherein the overall gain profile of the amplifier prior to use of a gain flattening filter varies by less than five decibels within the bandwidth of the plurality of signal wavelengths.
 132. A method of amplifying an optical signal having multiple wavelengths, the method comprising: amplifying a plurality of signal wavelengths using a plurality of cascaded Raman amplifier stages each having a gain profile, wherein the gain profile of at least some of the Raman stages is sloped; wherein at least two of the sloped gain profiles comprise approximately complimentary gain profiles, and wherein the combined effect of the gain profiles of the Raman stages contributes to an approximately flat overall gain profile over a plurality of signal wavelengths amplified by the amplifier; and wherein at least two of the Raman amplifier stages are coupled so as to cause longer pump wavelengths in one of the at least two Raman stages to accept power from shorter pump wavelengths in another of the at least two Raman stages.
 133. The method of claim 132, wherein amplifying a plurality of signal wavelengths using a plurality of cascaded Raman amplifier stages comprises: amplifying at least most of the plurality of signal wavelengths at a first Raman amplifier stage having a first sloped gain profile; and amplifying at least most of the plurality of signal wavelengths at a second Raman amplifier stage after those signal wavelengths have been amplified by the first stage, the second stage having a second sloped gain profile comprising an approximately complimentary gain profile to the first gain profile.
 134. The method of claim 132, wherein: the first sloped gain profile comprises a gain profile wherein a majority of longer signal wavelengths are amplified more than a majority of shorter signal wavelengths; and the second sloped gain profile comprises a gain profile wherein a majority of the shorter signal wavelengths are amplified more than a majority of the longer signal wavelengths.
 135. The method of claim 132, wherein: the first sloped gain profile comprises a gain profile wherein a majority of shorter signal wavelengths are amplified more than a majority of longer signal wavelengths; and the second sloped gain profile comprises a gain profile wherein a majority of the longer signal wavelengths are amplified more than a majority of the shorter signal wavelengths.
 136. A multi-stage optical amplifier, comprising: a first Raman amplifier stage having a first sloped gain profile operable to amplify a plurality of signal wavelengths, wherein the first sloped gain profile comprises a gain profile wherein a majority of shorter signal wavelengths are amplified more than a majority of longer signal wavelengths; and a second Raman amplifier stage having a second sloped gain profile operable to amplify at least most of the plurality of signal wavelengths after those wavelengths have been amplified by the first stage, wherein the second sloped gain profile comprises a gain profile wherein a majority of the longer signal wavelengths are amplified more than a majority of the shorter signal wavelengths; wherein the combined effect of the first and second Raman stages contributes to an approximately flat overall gain profile over the plurality of signal wavelengths.
 137. The amplifier of claim 136, wherein the overall gain profile of the amplifier prior to use of a gain flattening filter varies by less than five decibels within the bandwidth of the plurality of signal wavelengths.
 138. A method of amplifying an optical signal having multiple wavelengths, the method comprising: amplifying a plurality of signal wavelengths at a first Raman amplifier stage having a first sloped gain profile wherein a majority of shorter signal wavelengths are amplified more than a majority of longer signal wavelengths; and amplifying at least most of the plurality of signal wavelengths at a second Raman amplifier stage after those signal wavelengths have been amplified by the first stage, the second stage having a second sloped gain profile wherein a majority of the longer signal wavelengths are amplified more than a majority of the shorter signal wavelengths; wherein the combined effect of the first and second Raman stages contributes to an approximately flat overall gain profile over the plurality of signal wavelengths.
 139. The amplifier of claim 138, wherein the overall gain profile of the amplifier prior to use of a gain flattening filter varies by less than five decibels within the bandwidth of the plurality of signal wavelengths.
 140. A multi-stage optical amplifier, comprising: a first Raman amplifier stage having a first sloped gain profile operable to amplify a plurality of signal wavelengths; a second Raman amplifier stage having a second sloped gain profile operable to amplify at least most of the plurality of signal wavelengths after those wavelengths have been amplified by the first stage, the second sloped gain profile having an approximately complementary slope to the slope of the first sloped gain profile; wherein each of the Raman amplifier stages comprises a plurality of pump wavelength signals collectively operable to affect the slope and magnitude of the gain profile for that stage and wherein the longest pump wavelength comprises a wavelength at least ten (10) and no more than fifty (50) nanometers shorter than the shortest wavelength of the plurality of signal wavelengths; and wherein the combined effect of the first and second Raman stages contributes to an approximately flat overall gain profile over the plurality of signal wavelengths.
 141. The amplifier of claim 108, wherein the overall gain profile of the amplifier prior to use of a gain flattening filter varies by less than five decibels within the bandwidth of the plurality of signal wavelengths.
 142. The amplifier of claim 140, wherein the plurality of signal wavelengths comprise a bandwidth of at least forty (40) nanometers.
 143. The amplifier of claim 140, wherein the longest pump wavelength comprises a wavelength at least fifteen (15) and no more than fifty (50) nanometers shorter than the shortest wavelength of the plurality of signal wavelengths. 